How Chromosomes Determine Sex How Chromosomes Determine Sex Basic Genetics How Chromosomes Determine Sex A karyotype is the complete set of chromosomes of an organism. Humans have 46 chromosomes in total: 23 inherited from the mother (blue) and 23 from the father (pink). The sex chromosomes (labeled XY and XX) determine the individual's sex. Credit: PASIEKA/SPL/Getty Images By Regina Bailey How Chromosomes Determine Sex Chromosomes are long, segments of genes that carry heredity information. They are composed of DNA and proteins and are located within the nucleus of our cells . Chromosomes determine everything from hair color and eye color to sex. Whether you are a male or female depends on the presence or absence of certain chromosomes. Human cells contain 23 pairs of chromosomes for a total of 46. There are 22 pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes. The sex chromosomes are the X chromosome and the Y chromosome. Sex Chromosomes: In human sexual reproduction, two distinct gametes fuse to form a zygote. Gametes are reproductive cells produced by a type of cell division called meiosis . Gametes are also called sex cells .

They contain only one set of chromosomes and are said to be haploid . The male gamete, called the spermatozoan, is relatively motile and usually has a flagellum . The female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete. continue reading below our video Test Your General Science Knowledge When the haploid male and female gametes unite in a process called fertilization , they form what is called a zygote. The zygote is diploid , meaning that it contains two sets of chromosomes . Sex Chromosomes X-Y: The male gametes or sperm cells in humans and other mammals are heterogametic and contain one of two types of sex chromosomes . They are either X or Y. The female gametes or eggs however, contain only the X sex chromosome and are homogametic . The sperm cell determines the sex of an individual in this case. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male.

Sex Chromosomes X-O: Grasshoppers, roaches, and other insects have a similar system for determining the sex of an individual. Adult males lack a Y sex chromosome and have only an X chromosome. They produce sperm cells that contain either an X chromosome or no sex chromosome, which is designated as O. The females are XX and produce egg cells that contain an X chromosome. If an X sperm cell fertilizes an egg, the resulting zygote will be XX or female. If a sperm cell containing no sex chromosome fertilizes an egg, the resulting zygote will be XO or male. Sex Chromosomes Z-W: Birds, insects like butterflies, frogs , snakes , and some species of fish have a different system for determining sex. In these animals it is the female gamete that determines the sex of an individual. Female gametes can either contain a Z chromosome or a W chromosome. Male gametes contain only the Z chromosome. Females of these species are ZW and males are ZZ. Parthenogenesis: What about animals like most kinds of wasps, bees, and ants that have no sex chromosomes? How is sex determined? In these species, fertilization determines sex. If an egg becomes fertilized, it will develop into a female.

A non-fertilized egg may develop into a male. The female is diploid and contains two sets of chromosomes , while the male is haploid . This development of an unfertilized egg into a male and a fertilized egg into a female is a type of parthenogenesis known as arrhenotokous parthenogenesis. Chromosomes and Gender Chromosomes and Gender ï¿½ Mitosis Chromosomes are long, stringy aggregates of genes that carry heredity information. They are composed of DNA and proteins and are located within the nucleus of our cells. Chromosomes determine everything from hair color and eye color to gender. Whether you are a male or female depends on the presence or absence of certain chromosomes. Human cells contain 23 pairs of chromosomes for a total of 46. There are 22 pairs of autosomes and one pair of sex chromosomes. The sex chromosomes are the X chromosome and the Y chromosome. These chromosomes determine gender. Karyotype of a normal male with 22 pairs of autosomes and one pair of sex chromosomes. Image Credit: U.S. Department of Energy Human Genome Program, In human sexual reproduction, two distinct gametes fuse to form a zygote.

Gametes are reproductive cells produced by a type of cell division called meiosis . They contain only one set of chromosomes and are said to be haploid (one set of 22 autosomes and one sex chromosome). The male gamete, called the spermatozoan, is relatively motile and usually has a flagellum . The female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete. When the haploid male and female gametes unite in a process called fertilization, they form what is called a zygote. The zygote is diploid, meaning that it contains two sets of chromosomes (two sets of 22 autosomes and two sex chromosomes). Sex Chromosomes X-Y The male gametes or sperm cells in humans and other mammals are heterogametic and contain one of two types of sex chromosomes. They are either X or Y. The female gametes or eggs however, contain only the X sex chromosome and are homogametic . The sperm cell determines the sex of an individual in this case. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female.

If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male. Sex Chromosomes X-O Grasshoppers, roaches, and other insects have a similar system for determining the sex of an individual. Adult males lack a Y sex chromosome and have only an X chromosome. They produce sperm cells that contain either an X chromosome or no sex chromosome, which is designated as O. The females are XX and produce egg cells that contain an X chromosome. If an X sperm cell fertilizes an egg, the resulting zygote will be XX or female. If a sperm cell containing no sex chromosome fertilizes an egg, the resulting zygote will be XO or male. Sex Chromosomes Z-W Birds, insects like butterflies, and some species of fish have a different system for determining gender. In these animals it is the female gamete that determines the sex of an individual. Female gametes can either contain a Z chromosome or a W chromosome. Male gametes contain only the Z chromosome. Females of these species are ZW and males are ZZ. Parthenogenesis What about animals like most kinds of wasps, bees, and ants that have no sex chromosomes? How is gender determined? In these species, fertilization determines gender.

If an egg becomes fertilized it will develop into a female. A non-fertilized egg may develop into a male. The female is diploid and contains two sets of chromosomes, while the male is haploid. This development of an unfertilized egg into an individual is called parthenogenesis . WHO | Gender and Genetics Gender and Genetics Genetic Components of Sex and Gender Humans are born with 46 chromosomes in 23 pairs. The X and Y chromosomes determine a person’s sex. Most women are 46XX and most men are 46XY. Research suggests, however, that in a few births per thousand some individuals will be born with a single sex chromosome (45X or 45Y) (sex monosomies) and some with three or more sex chromosomes (47XXX, 47XYY or 47XXY, etc.) (sex polysomies). In addition, some males are born 46XX due to the translocation of a tiny section of the sex determining region of the Y chromosome. Similarly some females are also born 46XY due to mutations in the Y chromosome.

Clearly, there are not only females who are XX and males who are XY, but rather, there is a range of chromosome complements, hormone balances, and phenotypic variations that determine sex. The biological differences between men and women result from two processes: sex determination and differentiation.(3) The biological process of sex determination controls whether the male or female sexual differentiation pathway will be followed. The process of biological sex differentiation (development of a given sex) involves many genetically regulated, hierarchical developmental steps. More than 95% of the Y chromosome is male-specific (4) and a single copy of the Y chromosome is able to induce testicular differentiation of the embryonic gonad. The Y chromosome acts as a dominant inducer of male phenotype and individuals having four X chromosomes and one Y chromosome (49XXXXY) are phenotypically male. (5) When a Y chromosome is present, early embryonic testes develop around the 10th week of pregnancy. In the absence of both a Y chromosome and the influence of a testis-determining factor (TDF), ovaries develop. Gender, typically described in terms of masculinity and femininity, is a social construction that varies across different cultures and over time.

(6) There are a number of cultures, for example, in which greater gender diversity exists and sex and gender are not always neatly divided along binary lines such as male and female or homosexual and heterosexual. The Berdache in North America, the fa’afafine (Samoan for “the way of a woman”) in the Pacific, and the kathoey in Thailand are all examples of different gender categories that differ from the traditional Western division of people into males and females. Further, among certain North American native communities, gender is seen more in terms of a continuum than categories, with special acknowledgement of “two-spirited” people who encompass both masculine and feminine qualities and characteristics. It is apparent, then, that different cultures have taken different approaches to creating gender distinctions, with more or less recognition of fluidity and complexity of gender. XYY Males Case Example 1: Genetics as an important determinant of biological sex In 1959, chromosomal analysis of two human disorders, Turner syndrome and Klinefelter syndrome, demonstrated for the first time that genetic factors on the Y chromosomes of mammals are important determinants in male sex.

(7) The sex determining region on the Y chromosome, referred to as the SRY, has a fundamental role in sex determination and is believed to be the switch that initiates testis development. It is, therefore, responsible for the initiation of male sex determination during embryo development. (8) SRY-box-related (SOX) genes have been identified on autosomes (9) and mutations in the SRY or SOX gene have been implicated in sex reversal. (10) In other words, individuals with SRY mutations should develop as females. (11) However, it is likely that there are other sex determining genes involved in phenotypic sex. (12) The DNA sequence of the human X chromosome and male-specific region of the Y chromosome have recently been published, enabling further research into sex-specific genes. (13) Genetically Determined Disorders of Sexual Differentiation Typical sexual development is the result of numerous genes, and mutation in any of these genes can result in partial or complete failure of sex differentiation. These include mutations or structural anomalies of the SRY region on the Y chromosome resulting in XY gonadal dysgenesis, XX males, or XY females; defects of androgen biosynthesis or androgen receptors, and others.

Androgen Insensitivity Syndrome ELSI of Genetic Components of Sex and Gender The issues of gender assignment, gender verification testing, and legal definitions of gender are especially pertinent to a discussion on the ELSI of gender and genetics. These practices, however, are misnomers as they actually refer to biological sex and not gender. Such a discrepancy is highlighted by the existence of intersex individuals whose psychosexual development and gender sometimes do not match the biological sex assigned to them as infants. In this report the term ‘sex’ will be used where the practice refers to biological sex and not the more social construct of ‘gender’. Sex Chromosome Abnormalities Chromosomes are the structures that carry genes which in turn transmit hereditary characteristics from parents to offspring. Humans have 23 pairs of chromosomes, one half of each pair inherited from each parent. The Y chromosome is small, carries few genes, and has abundant repetitive sequence, while the X chromosome is more autosome-like in form and content. (14)Despite being relatively gene-poor overall due to reduced recombination, the X and Y sex chromosomes are enriched for genes that relate to sexual development.

(15) Aneuploidy is the condition of having less than (monosomy) or more than (polysomy) the normal diploid number of chromosomes. Aneuploidy occurs in at least 5% of all pregnancies and is the most commonly recognized chromosome abnormality in humans. (16) Divergence from the normal number of X and Y chromosomes, called sex chromosome aneuploidy (SCA), accounts for approximately half of all chromosomal anomalies in humans with a total frequency of 1:400. (17) Sex chromosome physical abnormalities are able to be diagnosed prenatally via amniocentesis and chorionic villi sampling (CVS). Prenatal diagnosis of SCA is increasing because of the widespread use of these technologies. (18) The high frequency of individuals with SCA is due to the fact that their effects are generally not as severe as autosomal abnormalities and are rarely lethal. Indeed, most cases of SCA are compatible with normal life expectancy and often go undiagnosed. (19) Still, it is estimated that1 in 3 miscarriages is due to aneuploidy affecting the foetus.

(20) The induced abortion rate for SCA has steadily decreased from 100% in the 1970s to 69% in the 1980s and 49% in the 1990s. (21) It has been suggested that this decrease is directly the result of improved knowledge of the conditions associated with SCA combined with the influence of genetic counseling. (22) Turner syndrome This disorder, also referred to as monosomy X (45X) occurs in individuals that have one X chromosome, no Y chromosome, and are phenotypically female. Although 45X is a frequent chromosomal anomaly, Turner syndrome is rare with a live-birth frequency of 1:3000, (23) as only 1 in 40 affected zygotes develops to term. (24) Affected individuals experience abnormal growth patterns, are short in stature, generally lack prominent female secondary sexual characteristics and are sterile. In some instances of Turner syndrome, there is slight mental retardation. XXX Females Women with three X chromosomes (47XXX) experience normal development of sexual traits and are fertile.

Affected individuals are usually taller than average and have slender builds. The frequency of women obtaining an extra X chromosome is approximately 1:1000. There is no severe phenotype associated with three X chromosomes in women. (25) These women may have slight learning difficulties. Klinefelter Syndrome Klinefelter syndrome (47XXY or XY/XXY mosaic) with male phenotype is the most pervasive sex chromosomal anomaly (26) affecting approximately 1:600 males. (27) Males with Klinefelter syndrome carry two or more X chromosomes which results in abnormal development of the testis, leading to hypogonadism and infertility. (28) Affected individuals are often tall and produce relatively small amounts of testosterone. As a result of this hormone imbalance, affected males have incompletely developed secondary male sex characteristics. XYY Males Men inheriting an additional Y chromosome are usually taller than average and are prone to acne because they produce higher than average levels of testosterone. Affected males are typically fertile and many are unaware that they have a chromosomal abnormality. The frequency of males born with an additional Y chromosome is approximately 1:1000.

Hermaphroditism ‘True’ hermaphroditism is a genetic condition in which affected individuals have both mature ovarian and testicular tissue. (29) There are no published population-wide estimates of the frequency of true hermaphrodites. (30) The autosomal inheritance of this condition suggests that genes controlling sexual development and differentiation are not limited to the sex chromosomes.(31) Blackless et al. suggest that such familial inheritance “opens the possibility that, as with other inherited forms of sexual ambiguity, there may be pockets, perhaps even large geographical regions, with relatively high frequencies of true hermaphroditism.” (32) Congenital Adrenal Hyperplasia Congenital adrenal hyperplasia (CAH) is an inherited autosomal recessive condition that can affect both boys and girls. It is the most common cause of intersexuality in females with 46XX, where untreated girls develop an outwardly male appearance. This disorder, also called adrenogenital syndrome (AGS), results from a genetically caused deficiency of cortisol, a steroid hormone produced by the adrenal cortex.

The disorder occurs with a frequency of 1:5000 and results in incomplete female sex differentiation and increased androgenic effects due to a compensatory increase in adrenocortical hormone (ACTH). (33) Androgen Insensitivity Syndrome Androgen Insensitivity Syndrome (AIS) is an X-linked recessive disorder in which affected individuals have external female genitalia and breast development despite being genetically male (46XY). Tissues of affected individuals are unresponsive to male hormones (androgens) yet respond to estrogens. Wisniewski et al. assessed the physical and psychosexual status of 14 women with complete androgen insensitivity syndrome (CAIS) by questionnaire as well as medical examination in order to determine the participants' knowledge of their disorder as well as their opinion of medical and surgical treatment. (34) Most of the women questioned expressed satisfaction with their psychosexual development and sexual function. All of the women who participated in the study were satisfied at having been raised as females, and none of the participants desired gender reassignment. (35) Gender Assignment of Intersex Infants and Children Intersex is defined as a congenital anomaly of the reproductive and sexual system.

An estimate about the birth prevalence of intersex is difficult to make because there are no concrete parameters to the definition of intersex. The Intersex Initiative, a North-American based organization, estimates that one in 2,000 children, or five children per day in the United States, are born visibly intersex. (36) This estimate sits within range; from genital anomalies, such as hypospadias, with a birth prevalence of around 1:300 to complex genital anomalies in which sex assignment is difficult, with a birth prevalence of about 1:4500. (37) Many intersex children have undergone medical intervention for health reasons as well as for sociological and ideological reasons. An important consideration with respect to sex assignment is the ethics of surgically altering the genitalia of intersex children to “normalize” them. Clitoral surgery for intersex conditions was promoted by Hugh Hampton Young in the United States in the late 1930s. Subsequently, a standardized intersex management strategy was developed by psychologists at Johns Hopkins University (USA) based on the idea that infants are gender neutral at birth. (38) Minto et al.

note that “the theory of psychosexual neutrality at birth has now been replaced by a model of complex interaction between prenatal and postnatal factors that lead to the development of gender and, later, sexual identity”. (39) However, currently in the United States and many Western European countries, the most likely clinical recommendation to the parents of intersex infants is to raise them as females, often involving surgery to feminize the appearance of the genitalia. (40) Minto et al. conducted a study aiming to assess the effects of feminizing intersex surgery on adult sexual function in individuals with ambiguous genitalia. As part of this study, they noted a number of ethical issues in relation to this surgery, including that: there is no evidence that feminizing genital surgery leads to improved psychosocial outcomes; feminizing genital surgery cannot guarantee that adult gender identity will develop as female; and that adult sexual function might be altered by removal of clitoral or phallic tissue. (41) Developmental biology suggests that a strict belief in absolute sexual dimorphism is incorrect. (42) Instead, Blackless et al. suggest two overlapping bell-shaped curves to conceptualize sexual variations across populations.

Qualitative variation in chromosome complement, genital morphology and hormonal activity falls under the area of overlap. (43) Such an opinion challenges the need for medical intervention in cases of intersexuality. Legal Definitions of Gender An example of the implications of legal definitions of sexual affiliation In addition to social implications, sex and gender categorization has important political and legal implications as well. The identification of an individual as either biologically male or female can have legal ramifications for marriage licenses, spousal support and eligibility for parenthood. The issue of sexual classification, however, is complicated by factors such as chromosomal complement, external genitalia, gender identification and surgical alteration. In the United States, as in most countries, there is an absence of controlling legislation that defines exactly how a person’s gender is determined. (44) The following example demonstrates some of the complexities this may raise. In 1999, the Texas Court of Appeals considered the validity of a marriage between a man and a person born genetically as a man, but surgically altered to have the physical characteristics of a woman.

(45) As in most American states, Texas family law prohibits same-sex marriages, and the United States Congress has passed the Defense of Marriage Act that, for federal purposes, defines marriage as a “legal union between one man and one woman”. (46) Despite the fact that medical experts gave testimony to the effect that the individual in question was psychologically and psychiatrically female before and after the sex reassignment surgery, the court held that chromosome complement, and not sex reassignment surgery, outward gender characteristics, nor psychosocial self-identification, determine a person’s gender. The court invalidated the marriage on the grounds that the transsexual woman was legally a man. (47) This case is one example of how the legal system in one country has dealt with the complexities of defining sex and gender and the implications of defining gender and sex. Clearly, there are many other potential legal implications of such definitions and the complicated issues they raise are many. 2 Sex chromosomes determine gender a Human females have two X - BIO - 03 View Full Document 2. Sex chromosomes determine gender. a. Human females have two X chromosomes. b. Males have one X and one Y. c.

In some animals the male has two identical sex chromosomes, not the females. 3. The remaining chromosomes, autosomes, have the same length, shape, and centromere location. Humans have 44 autosomes and two sex chromosomes. B. Sex Determination 1. Each human egg will contain 22autosomes plus one X, but sperm will carry 22autosomes plus either an X or a Y. a. An X-bearing egg plus an X-bearing sperm produces female offspring. b. An X-bearing egg plus a Y-bearing sperm produces male offspring. 2. The Y chromosome carries a male-determining (SRY) gene, which leads to formation of the testes, which secrete testosterone. Absence of the male gene in females results in formation of ovaries. 3. The X chromosome obviously codes for sexual traits, but it also carries many genes for nonsexual traits. C. Karyotyping 1. A karyotype is a diagnostic tool that is a preparation of an individual’s metaphase chromosomes. 2. After mitosis is arrested in metaphase, the chromosomes are photographed, then cut out and arranged by pairs according to size, shape, and length. 3.

This array can be compared to a normal standard and analyzed for extra or missing chromosomes. 11.2. Examples of Humand Inheritance Patterns A. Autosomal Dominant Inheritance 1. The dominant allele is nearly always expressed even in heterozygotes. 2. If one parent is heterozygous and the other homozygous recessive, there is a 50 percent chance that any child will be heterozygous. 3. If the gene (and its resulting disorder) reduces the chance of surviving or reproducing, its frequency should decrease, but may not due to mutations, nonreproductive effects, and postreproductive onset. a. Achondroplasia (dwarfism) is a benign abnormality that does not affect persons to the point that reproduction is impossible, so the gene is passed on. b. Huntington disease is a serious degeneration of the nervous system with an onset from age 40 onward, by which time the gene has (usually) been passed from parent to offspring unknowingly. This preview has intentionally blurred sections. Sign up to view the full version. B. Autosomal Recessive Inheritance 1. Characteristics of this condition: a.

Either parent can carry the recessive allele on an autosome. b. Heterozygotes are symptom-free; homozygotes are affected. This is the end of the preview. Sign up to access the rest of the document. TERM TAKE HOME QUIZZ # 6. ON CHAPTER 12 (40 Points) DNA STRUCTURE AND FUNCTION Multiple-Ch CH 12 HW Human Chromosomal Abnormalities: Sex Chromosome Abnormalities Sex Chromosome Abnormalities The majority of known types of chromosomal abnormalities involve sex chromosomes . In frequency of occurrence, they are only slightly less common than autosomal abnormalities. However, they are usually much less severe in their effects. The high frequency of people with sex chromosome aberrations is partly due to the fact that they are rarely lethal conditions. Like Down syndrome and other autosomal problems, sex chromosome gross abnormalities can be diagnosed before birth by amniocentesis and chorionic villi sampling. Sex chromosome abnormalities are gender specific. Normal males inherit an X and a Y chromosome while females have two X's. A single Y chromosome is sufficient to produce maleness while its absence is necessary for femaleness. Female abnormalities are due to variations in the number of X chromosomes.

Male abnormalities are the result of irregular numbers of either the X or the Y chromosome or both. Female Sex Chromosome Abnormalities Turner syndrome occurs when females inherit only one X chromosome--their genotype is X0 (i.e., monosomy X). If they survive to birth, these girls have abnormal growth patterns. They are short in stature, averaging 4 foot 7 inches as adults, and often have distinctive webbed necks (i.e., extra folds of skin), small jaws, and high arched palates. They generally lack prominent female secondary sexual characteristics. They have exceptionally small, widely spaced breasts, broad shield-shaped chests, and turned-out elbows. Their ovaries do not develop normally and they do not ovulate. The few o�cytes that they produce are destroyed by the time they are two. They are in a sense postmenopausal from early childhood and are sterile. However, they can become pregnant and give birth if fertilized eggs from a donor are implanted. Women with Turner syndrome have a higher than average incidence of thyroid disease, vision and hearing problems, heart defects, diabetes , and other autoimmune disorders . In a few individuals, there is slight mental retardation. Turner syndrome is rare.

Current estimates of its frequency range from 1 in 2,000 to 1 in 5,000 female infants. If diagnosed in early childhood, regular injections of human growth hormones can increase their stature by a few inches. Beginning around the normal age of puberty, estrogen replacement therapy can result in some breast development and menstruation. These treatments allow Turner syndrome women to appear relatively normal. Turner Syndrome --characteristics of Turner Syndrome and treatment for it. This link takes you to an external website. To return here, you must click the "back" button on your browser program. (length = 2 mins, 39 secs) Triple-X syndrome occurs in women who inherit three X chromosomes--their genotype is XXX or more rarely XXXX or XXXXX. As adults, these "super-females" or "metafemales" , as they are sometimes known, generally are an inch or so taller than average with unusually long legs and slender torsos but otherwise appear normal. They usually have normal development of sexual characteristics and are fertile but tend to have some ovary abnormalities that can lead to premature ovarian failure.

They may have slight learning difficulties, especially in speech and language skills, and are usually in the low range of normal intelligence (especially the XXXX and XXXXX individuals). They frequently are very tall in childhood and tend to be emotionally immature for their size. This sometimes results in teachers and other adults labeling them as troublemakers because they expect more maturity from bigger girls. However, they are usually as emotionally mature as other girls of their age. None of these traits prevent them from being socially accepted as ordinary adult women. Individuals who are genetic mosaics (XX/XXX) have less noticeable symptoms. Triple-X syndrome is less rare than Turner syndrome, but little is known about it. The frequency is approximately 1 in 1,000 female infants and it occurs more commonly when the mother is older. Male Sex Chromosome Abnormalities Klinefelter syndrome males inherit one or more extra X chromosomes--their genotype is XXY or more rarely XXXY or XY/XXY mosaic. In severe cases, they have relatively high-pitched voices, asexual to feminine body contours as well as breast enlargement, and comparatively little facial and body hair. They are sterile or nearly so, and their testes and prostate gland are small.

As a result, they produce relatively small amounts of testosterone . The feminizing effects of this hormonal imbalance can be significantly diminished if Klinefelter syndrome boys are regularly given testosterone from the age of puberty on. Like triple-X females (described above), many Klinefelter syndrome men are an inch or so above average height. They also are likely to be overweight. They usually have learning difficulties as children, especially with language and short-term memory. If not given extra help in early childhood, this often leads to poor school grades and a subsequent low self esteem. However, most men who have Klinefelter syndrome are sufficiently ordinary in appearance and mental ability to live in society without notice. It is not unusual for Klinefelter syndrome adults with slight symptoms to be unaware that they have it until they are tested for infertility. They are usually capable of normal sexual function, including erection and ejaculation, but many, if not most, are unable to produce sufficient amounts of sperm for conception. Klinefelter syndrome males with more than two X chromosomes usually have extreme symptoms and are often slightly retarded mentally. Men who are mosaic (XY/XXY) generally have the least problems.

There is no evidence that Klinefelter syndrome boys and men are more inclined to be homosexual, but they are more likely to be less interested in sex. They have a higher than average risk of developing osteoporosis , diabetes, and other autoimmune disorders that are more common in women. This may be connected to low testosterone production. Subsequently, regular testosterone therapy is often prescribed. The frequency of Klinefelter syndrome has been reported to be between 1 in 500 and 1 in 1000 male births. This makes it one of the most common chromosomal abnormalities. Males with Down syndrome sometimes also have Klinefelter syndrome. Both syndromes are more likely to occur in babies of older mothers. Klinefelter Syndrome --traits described by a doctor of medical genetics This link takes you to an external website. To return here, you must click the "back" button on your browser program. (length = 1 mins, 48 secs) XYY syndrome males inherit an extra Y chromosome--their genotype is XYY. As adults, these "super-males" are usually tall (above 6 feet) and generally appear and act normal. However, they produce high levels of testosterone.

During adolescence, they often are slender, have severe facial acne, and are poorly coordinated. They are usually fertile and lead ordinary lives as adults. Many, if not most, are unaware that they have a chromosomal abnormality. The frequency of XYY syndrome is not certain due to statistical differences between different studies. It may be as common as 1 in 900 male births to as rare as 1 in 1500 or even 1 in 2,000. XYY syndrome is also referred to as Jacobs syndrome. Early studies of XYY syndrome done in European prisons initially led to the erroneous conclusion that these men were genetically predisposed to antisocial, aggressive behavior, below average intelligence, and homosexuality. Contributing to the early view that XYY syndrome men have serious personality disorders was the case of Richard Speck. In 1966, he coldly murdered 8 nurses in a Chicago dormitory. At his trial, his lawyer claimed that he was innocent due to uncontrollable urges caused by his XYY genotype. This novel appeal was akin to claiming insanity or severely diminished mental competence. The jury was not convinced and found him guilty of murder. He was sentenced to life in prison where he eventually died.

In fact, Richard Speck did not have an XYY genotype. However, some researchers suggest that the high testosterone levels of XYY men can make them somewhat more prone to violence and that this may cause higher rates of wife beating. Y chromosome - Genetics Home Reference Y chromosome Description The Y chromosome is one of the two sex chromosomes in humans (the other is the X chromosome). The sex chromosomes form one of the 23 pairs of human chromosomes in each cell. The Y chromosome spans more than 59 million building blocks of DNA (base pairs) and represents almost 2 percent of the total DNA in cells. Each person normally has one pair of sex chromosomes in each cell. The Y chromosome is present in males, who have one X and one Y chromosome, while females have two X chromosomes. Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. The Y chromosome likely contains 50 to 60 genes that provide instructions for making proteins. Because only males have the Y chromosome, the genes on this chromosome tend to be involved in male sex determination and development.

Sex is determined by the SRY gene, which is responsible for the development of a fetus into a male. Other genes on the Y chromosome are important for male fertility. Many genes are unique to the Y chromosome, but genes in areas known as pseudoautosomal regions are present on both sex chromosomes. As a result, men and women each have two functional copies of these genes. Many genes in the pseudoautosomal regions are essential for normal development. Related Information 46,XX testicular disorder of sex development In most individuals with 46,XX testicular disorder of sex development, the condition results from an abnormal exchange of genetic material between chromosomes (translocation). This exchange occurs as a random event during the formation of sperm cells in the affected person's father. The translocation affects the gene responsible for development of a fetus into a male (the SRY gene). The SRY gene, which is normally found on the Y chromosome, is misplaced in this disorder, almost always onto an X chromosome. A fetus with an X chromosome that carries the SRY gene will develop as a male despite not having a Y chromosome.

47,XYY syndrome Males with 47,XYY syndrome have one X chromosome and two Y chromosomes in each cell, for a total of 47 chromosomes. It is unclear why an extra copy of the Y chromosome is associated with tall stature, learning problems, and other features in some boys and men. Some males with 47,XYY syndrome have an extra Y chromosome in only some of their cells. This phenomenon is called 46,XY/47,XYY mosaicism. 48,XXYY syndrome 48,XXYY syndrome is caused by the presence of an extra X chromosome and an extra Y chromosome in a male's cells. Extra genetic material from the X chromosome interferes with male sexual development, preventing the testes from functioning normally and reducing the levels of testosterone (a hormone that directs male sexual development) in adolescent and adult males. Extra copies of genes from the pseudoautosomal region of the extra X and Y chromosome contribute to the signs and symptoms of 48,XXYY syndrome; however, the specific genes have not been identified.

Y chromosome infertility Y chromosome infertility is usually caused by deletions of genetic material in regions of the Y chromosome called azoospermia factor (AZF) A, B, or C. Genes in these regions are believed to provide instructions for making proteins involved in sperm cell development, although the specific functions of these proteins are unknown. Deletions in the AZF regions may affect several genes. The missing genetic material likely prevents production of a number of proteins needed for normal sperm cell development, resulting in an inability to father children. Chromosomes Fact Sheet - National Human Genome Research Institute (NHGRI) National Human Genome Research Institute Enter word(s) to search Definitions for genetic terms used on this page. En Español: How were chromosomes discovered? What is a chromosome? Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique. The term chromosome comes from the Greek words for color (chroma) and body (soma).

Scientists gave this name to chromosomes because they are cell structures, or bodies, that are strongly stained by some colorful dyes used in research. Top of page What do chromosomes do? The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones. Without such packaging, DNA molecules would be too long to fit inside cells. For example, if all of the DNA molecules in a single human cell were unwound from their histones and placed end-to-end, they would stretch 6 feet. For an organism to grow and function properly, cells must constantly divide to produce new cells to replace old, worn-out cells. During cell division, it is essential that DNA remains intact and evenly distributed among cells. Chromosomes are a key part of the process that ensures DNA is accurately copied and distributed in the vast majority of cell divisions. Still, mistakes do occur on rare occasions. Changes in the number or structure of chromosomes in new cells may lead to serious problems. For example, in humans, one type of leukemia and some other cancers are caused by defective chromosomes made up of joined pieces of broken chromosomes. It is also crucial that reproductive cells, such as eggs and sperm, contain the right number of chromosomes and that those chromosomes have the correct structure.

If not, the resulting offspring may fail to develop properly. For example, people with Down syndrome have three copies of chromosome 21, instead of the two copies found in other people. Top of page Do all living things have the same types of chromosomes? Chromosomes vary in number and shape among living things. Most bacteria have one or two circular chromosomes. Humans, along with other animals and plants, have linear chromosomes that are arranged in pairs within the nucleus of the cell. The only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome. When two reproductive cells unite, they become a single cell that contains two copies of each chromosome. This cell then divides and its successors divide numerous times, eventually producing a mature individual with a full set of paired chromosomes in virtually all of its cells. Besides the linear chromosomes found in the nucleus, the cells of humans and other complex organisms carry a much smaller type of chromosome similar to those seen in bacteria. This circular chromosome is found in mitochondria, which are structures located outside the nucleus that serve as the cell's powerhouses. Scientists think that, in the past, mitochondria were free-living bacteria with the ability to convert oxygen into energy.

When these bacteria invaded cells lacking the power to tap into oxygen's power, the cells retained them, and, over time, the bacteria evolved into modern-day mitochondria. Top of page What are centromeres? The constricted region of linear chromosomes is known as the centromere. Although this constriction is called the centromere, it usually is not located exactly in the center of the chromosome and, in some cases, is located almost at the chromosome's end. The regions on either side of the centromere are referred to as the chromosome's arms. Centromeres help to keep chromosomes properly aligned during the complex process of cell division. As chromosomes are copied in preparation for production of a new cell, the centromere serves as an attachment site for the two halves of each replicated chromosome, known as sister chromatids. Top of page What are telomeres? Telomeres are repetitive stretches of DNA located at the ends of linear chromosomes. They protect the ends of chromosomes in a manner similar to the way the tips of shoelaces keep them from unraveling. In many types of cells, telomeres lose a bit of their DNA every time a cell divides.

Eventually, when all of the telomere DNA is gone, the cell cannot replicate and dies. White blood cells and other cell types with the capacity to divide very frequently have a special enzyme that prevents their chromosomes from losing their telomeres. Because they retain their telomeres, such cells generally live longer than other cells. Telomeres also play a role in cancer. The chromosomes of malignant cells usually do not lose their telomeres, helping to fuel the uncontrolled growth that makes cancer so devastating. How many chromosomes do humans have? Humans have 23 pairs of chromosomes, for a total of 46 chromosomes. In fact, each species of plants and animals has a set number of chromosomes. A fruit fly, for example, has four pairs of chromosomes, while a rice plant has 12 and a dog, 39. Top of page How are chromosomes inherited? In humans and most other complex organisms, one copy of each chromosome is inherited from the female parent and the other from the male parent. This explains why children inherit some of their traits from their mother and others from their father. The pattern of inheritance is different for the small circular chromosome found in mitochondria.

Only egg cells - and not sperm cells - keep their mitochondria during fertilization. So, mitochondrial DNA is always inherited from the female parent. In humans, a few conditions, including some forms of hearing impairment and diabetes, have been associated with DNA found in the mitochondria. Top of page Do males have different chromosomes than females? Yes, they differ in a pair of chromosomes known as the sex chromosomes. Females have two X chromosomes in their cells, while males have one X and one Y chromosome. Inheriting too many or not enough copies of sex chromosomes can lead to serious problems. For example, females who have extra copies of the X chromosome are usually taller than average and some have mental retardation. Males with more than one X chromosome have Klinefelter syndrome, which is a condition characterized by tall stature and, often, impaired fertility. Another syndrome caused by imbalance in the number of sex chromosomes is Turner syndrome. Women with Turner have one X chromosome only. They are very short, usually do not undergo puberty and some may have kidney or heart problems. Top of page How were chromosomes discovered? Scientists looking at cells under the microscope first observed chromosomes in the late 1800s.

However, at the time, the nature and function of these cell structures were unclear. Researchers gained a much better understanding of chromosomes in the early 1900s through Thomas Hunt Morgan's pioneering studies. Morgan made the link between chromosomes and inherited traits by demonstrating that the X chromosome is related to gender and eye color in fruit flies. Meiosis | HHMI BioInteractive More About Meiosis Background At a genetic level, sexual reproduction is all about mixing up genes and putting together new combinations. The first step in this process is meiosis, a special form of cell division that produces gametesóthe egg and sperm cells. Most of the action during meiosis centers on the chromosomes . Cells in the human body have 46 chromosomes, including 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males). Because there are two sets of chromosomes, one from each parent, the cells are considered diploid. Meiosis starts with a diploid cell and turns it into four haploid cells, cells with only one set of chromosomes. This means that when the chromosomes of egg and sperm cells combine at fertilization, the embryo regains the normal diploid number. Meiosis mixes up the parental genes in two ways.

First, the members of each chromosome pair come together and swap segments in a process known as crossing over, or recombination . Second, because each gamete gets only half the parental chromosomes, the exact combination in each egg or sperm can and does vary. This is because during meiosis the chromosomes assort independently, with a random member of each pair going to each daughter cell. Because males have one X and one Y chromosome, half the cells get an X and half get a Y during the meiosis that leads to sperm production. (In females, all the eggs will get one or the other X.) In a general sense, the sex of the offspring is determined by the particular sex chromosome carried by the sperm. However, in the early weeks of development, all fetuses have preliminary structures for both sexes, and the immature gonads can become either testes or ovaries. In the seventh week of fetal development, a gene on the Y chromosome, if present, activates, and the bipotential gonads commit to becoming testes. In the absence of a Y chromosome, and the signal to form testes, the fetus develops as a girl. At least that's the way it usually happens.

In rare cases, an XX individual becomes a male or an XY individual becomes female. Researchers realized that studying the genes of these sex-reversed people could lead them to the master switch for sex determination. They subsequently identified a gene called SRY (sex-determining region on the Y chromosome). Introduction Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome. But what is it about the X or Y that determines sex? Before a meiotic cell divides, its two sets of chromosomes come together and cross over, or swap, segments. The first animation shows normal crossing over, where the X and Y chromosomes exchange pieces only at their tips. The second animation shows a rare mistake in which the Y chromosome transfers a gene called SRY to the X chromosome, resulting in sex-reversed babies. Studies of sex-reversed individuals led researchers to identify the master switch for sex determination, the SRY gene, which tells a fetus to become a boy. Part 1: Normal male meiosis In the cell nucleus, chromosomes contributed by this male's mother (in red) and father (in blue) pair up.

For clarity, only the X and Y sex chromosomes and 5 of the 22 pairs of autosomes (nonsex chromosomes) are shown. Each chromosome has replicated and consists of two identical chromatids. Crossing over can occur anywhere along the autosomes, and here, they swap segments at each end. The X and Y chromosomes normally cross over only at their tips (indicated in blue on the Y). Note that SRY lies below this region. The nuclear membrane breaks down, and the chromosomes line up along the cell's equatorial plane and then move to the poles. A random member of each chromosome pair goes to each haploid daughter cell. A second division separates the chromatids and produces four cells, which develop into sperm. In the top panel, a sperm with an X chromosome fertilizes the egg; in the bottom panel, a sperm with a Y chromosome fertilizes the egg. The XX and XY fetuses develop along the same pathway through week six. Then SRY switches on; the XY fetus develops into a boy (bottom panel), and the XX fetus becomes a girl (top panel). Part 2: Atypical male meiosis resulting in sex-reversed individuals Meiosis begins just as in the previous example.

However, this time the Y chromosome breaks below SRY, transferring SRY to the X chromosome. This produces two sperm with abnormal sex chromosomes. When they fertilize eggs, the XX (SRY+) embryo develops into a boy (top panel), and the XY (SRYñ) embryo develops into a girl. They are sex reversed. Learn More: Autosomes Autosomes are chromosomes other than sex chromosomes and are the same in both sexes. Learn More: Chromosomes Mammalian chromosomes are DNA molecules bound up with proteins, particularly proteins known as histones, which form a core for the strand of DNA to wrap around. Chromosomes become visible in the cell nucleus only during cell division when they are condensed into tightly coiled rods. They typically have two arms on either side of a centromere, a condensed region critical for the movement and sorting of chromosomes during cell division. The two halves of a replicated chromosome are called chromatids. Learn More: Crossing Over, or Recombination During the first meiotic division, homologous chromosomes synapse, or pair up. At this point, each chromosome consists of two identical chromatids.

Crossing over is a precise mechanism for cutting through the DNA of two chromatids and exchanging equivalent pieces without loss of information. In this way, the chromosomes transmitted to gametes can acquire mixtures of maternal and paternal genes. Learn More: Genes Genes are lengths of DNA that code for proteins and are the basic units of heredity. Different types of the same gene are called alleles and are responsible for variation in inherited traits. Each gene can be mapped to a specific location on a chromosome, and the proximity of different genes determines their linkage, or the likelihood that they will be inherited together. Recombination is more likely to separate alleles that are further apart on a chromosome than those with little space for crossovers between them. Learn More: Stages of Meiosis Meiosis I: During the first meiotic division, recombination occurs and the chromosome number is halved. Prophase I: Chromosomes condense and become visible. Homologous chromosomes pair up and recombination (crossing over) occurs. Crossovers may be visible as chiasmata, x-shaped connections between chromatids. Metaphase I: Paired chromosomes line up along the cell's equatorial plane.

Anaphase I: Homologous pairs separate and move to opposite poles. Telophase I: Chromosomes are at poles; nuclear membranes may re-form. Meiosis II: The second meiotic division closely resembles mitosis (the type of cell division that occurs in body cells), except that the starting and ending cells are haploid. Prophase II, metaphase II, anaphase II: The chromosomes again move to the equatorial plane, and this time the chromatids separate to opposite poles. Telophase II: Nuclear membranes re-form around the chromosomes. Learn More: Sex Chromosomes Sex chromosomes differ between the sexes and are involved in sex determination, although they may have other functions as well. Learn More: SRY (sex-determining region on the Y chromosome) Before the SRY gene was identified, scientists knew that there was a testes-determining factor on the Y chromosome. The challenge was to pinpoint its location. This was done by comparing first the observable physical structure of the chromosomes and second, when the technology allowed, the DNA sequences of sex-reversed individuals with those of the normal population.

(Sex reversal occurs in about 1 out of 20,000 births.) Screening with Y-specific DNA (DNA that is found only on the Y chromosome) showed that XY females tended to be missing a certain segment of DNA on the short arm of the Y chromosome, whereas XX males carried DNA from that same region. Mapping that region yielded SRY. The protein encoded by SRY is apparently a transcription factor, and thus it regulates the function of another gene or genes. Resources

Question: A human with two X sex chromosomes is usually which gender?
Answer: Female